elf: Turn _dl_printf, _dl_error_printf, _dl_fatal_printf into functions

elf: Turn _dl_printf, _dl_error_printf, _dl_fatal_printf into functions

This change makes it easier to set a breakpoint on these calls.

This also addresses the issue that including <ldsodefs.h> without
<unistd.h> does not result usable _dl_*printf macros because of the
use of the STD*_FILENO macros there.

(The private symbol for _dl_fatal_printf will go away again
once the exception handling implementation is unified between
libc and ld.so.)

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>